1. Field of the Invention
The present invention relates to an infrared image-processing apparatus for extracting an object by binarization processing of an image picked up by an infrared camera.
2. Description of the Related Art
Conventionally, when an object such as a pedestrian or a vehicle is extracted from an image picked up by an infrared camera, binarization processing is carried out in order to distinguish the object image from a background image. Binarization processing of an image distinguishes the object image from the background image included in the image captured by the infrared camera. The distinguishing of the object image from the background image is performed by setting a threshold brightness value which separates two peaks (background peak and object peak) obtained in the brightness histogram as the peak frequency of pixels appearing at a relatively low brightness range as background and at relatively high brightness range as the object.
A conventional example of an infrared image-processing apparatus which carries out binarization processing of the infrared image by setting a threshold brightness value between the background peak and the object peaks obtained on the brightness histogram of the infrared image is disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-160144. This document discloses a conventional image-processing device, which prepares a histogram from two images captured by a stereo camera as a function of the brightness value I shown on the abscissa and a frequency of pixels H [I] having the brightness value I on the vertical line, two brightness values corresponding to the object peaks and the background peaks are obtained, and the contrast values of two images picked up by the stereo camera are compared. One of the two images having a higher contrast value is selected as a reference image, and binarization processing is performed for the reference image using the threshold value of the selected image and an object is extracted from the binarized image.
The aforementioned image-processing apparatus makes it possible to avoid the failure in determining the object because of the differences of the contrast between two images picked up by the stereo camera.
However, when peak positions of the background peak and the object peak in the brightness histogram of the infrared images change because of a change of the amount of infrared radiation from the object, or because of a change in the season, weather, time of day, and ambient temperature, the problems arise in the aforementioned conventional infrared processing apparatus in that the object cannot be extracted accurately or something other than the object is extracted.
The present invention is made to solve the above problems and to provide an infrared image-processing apparatus capable of accurately extracting an object in response to a change in the brightness histograin of the infrared image captured by the infrared camera.
A first aspect of the present invention provides an infrared image-processing apparatus, which executes a binarization process for an infrared image captured by an infrared camera by setting a threshold value (for example, the threshold value ITH in the embodiment) in between two brightness values corresponding to two peak frequencies appearing in the brightness histogram, which are respectively connected to a background image and an object image, comprising a brightness threshold value setting section (for example, step S4 in the first and second embodiments, steps S31-S45 in the first embodiment, and steps S46 to S49 in the second embodiment) for changing said threshold value such that the area ratio of the object image extracted by said binarization process is set in a predetermined range.
By constituting the infrared image-processing apparatus as described above, it becomes possible for the present infrared image-processing apparatus to extract an object by the binarization process without being affected by the noise of the background image by changing the threshold value for separating the object such that the area ratio of the object image is set in a predetermined area range.
According to the second aspect of the present invention, in the above infrared image-processing apparatus, said brightness value setting section comprises in advance a lower limit, that is, a minimum brightness value, which is above the brightness value corresponding to the peak frequency associated with the background image in the brightness-frequency histogram, and an upper limit, that is, a maximum brightness value, which is below the brightness value corresponding to the peak frequency associated with the object image in the brightness frequency histogram.
By constituting the infrared image-processing apparatus as described above, the binarization process makes it possible to extract the object image without fail, eliminating the chance of failing to extract the object image, i.e., by changing the threshold value limiting the object area ratio to be within a range between the minimum brightness value and the maximum brightness value when the threshold value is changed such that the object area ratio is within a predetermined range.